Can global warming (weirding) lead to smaller mammals?
This, hot off the press in tomorrow’s issue of Science:

Body size plays a critical role in mammalian ecology and physiology. Previous research has shown that many mammals became smaller during the Paleocene-Eocene Thermal Maximum (PETM), but the timing and magnitude of that change relative to climate change have been unclear. A high-resolution record of continental climate and equid body size change shows a directional size decrease of ~30% over the first ~130,000 years of the PETM, followed by a ~76% increase in the recovery phase of the PETM. These size changes are negatively correlated with temperature inferred from oxygen isotopes in mammal teeth and were probably driven by shifts in temperature and possibly high atmospheric CO2 concentrations. These findings could be important for understanding mammalian evolutionary responses to future global warming.

The researchers conclude:

PETM warming was similar in magnitude to that predicted by some global models over the next century (1) but occurred at a much slower rate and began from a warmer late Paleocene baseline. Nevertheless, some generalizations applicable to future warming may still be relevant. Diminished body size in some mammal species, along with changes in ecology and physiology, might be expected in response to warming. The pattern of dwarfing seen in the PETM mirrors recent reductions in body size in endotherms that have been attributed to anthropogenic warming (10, 12). Although the rate of present warming is much faster than during the PETM, and mammals may not respond in exactly the same manner, the dramatic response to warming observed in PETM equids provides a measure of possible responses to future warming in modern mammals.